Threshold voltage scalable buffer with reference level

ABSTRACT

A buffer circuit ( 10 ). The buffer circuit ( 10 ) includes a first inverter ( 12 ) with a first current limiter ( 18 ) that limits the standby current used by the first inverter ( 12 ). Further, the buffer circuit ( 10 ) includes a second inverter ( 14 ) that is coupled to an output of the first inverter ( 12 ). The input buffer ( 10 ) converts a first logic level of an input signal provided to the first inverter ( 12 ) to a second logic level at an output of the second inverter ( 14 ). The buffer circuit ( 10 ) also includes a second current limiting circuit ( 16 ) that is coupled between the first and second inverters ( 12  and  14 ) to further limit the standby current in the buffer circuit ( 10 ).

This application is a Continuation of U.S. Ser. No. 09/137,206, filedAug. 20, 1998 now U.S. Pat. No. 5,945,844, which is a Continuation ofU.S. Ser. No. 08/918,623, filed Aug. 22, 1997, now U.S. Pat. No.5,861,763, which is a Continuation of U.S. Ser. No. 08/648,443, filedMay 15, 1996, now U.S. Pat. No. 5,703,500.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to integrated circuits and, inparticular, to an input buffer.

BACKGROUND OF THE INVENTION

Electronic systems typically are produced from component parts that areindependently designed and manufactured. For example, a typical personalcomputer includes a microprocessor, some type of memory device such as adynamic random access memory and a number of other integrated circuits.These components are designed according to standards for variousfamilies of integrated circuits. Integrated circuits in a “family” aredesigned to recognize defined voltage levels as high and low logicvalues used by the circuits to communicate and process data. Forexample,the “transistor-transistor logic” or “TTL” family of integratedcircuits typically recognize 0.7 to 0.8 volts as a low logic value and2.0 to 2.4 volts as a high logic value. In the “complimentarymetal-oxide serniconductor” or “CMOS” family, circuits use a voltagethat is approximately equal to the power supply as a high logic leveland ground potential as the low logic level. During normal operation ofthe electronic systems, the components communicate with each other toshare data and control signals. To do so, the components often need torecognize logic levels for a different family of integrated circuits.Designers have developed various buffer circuits that convert logiclevels from a circuit in one family to logic levels of a differentfamily.

One type of buffer circuit converts TmL logic levels for use by a CMOScircuit. The buffer typically consists of two CMOS inverters coupled inseries. Unfortunately, this design draws a significant standby current.In an integrated circuit, such as a dynamic random access memory, thathas a number of inputs, the standby current could decrease the overallperformance of the integrated circuit. With the input buffer inactive-standby mode for long periods of time, this problem isexacerbated. Other buffers include Schmitt triggers. The schmitt triggersimilarly draws a significant standby current.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art forbuffer circuit that operates with an insignificant standby current.

SUMMARY OF THE INVENTION

The above-mentioned problems with buffer circuits and other problems areaddressed by the present invention which will be understood by readingand studying the following specification. A buffer circuit is describedwhich converts logic levels with reduced standby current.

More particularly, in one embodiment the present invention provides abuffer circuit that includes first and second inverters. The firstinverter includes a first current limiter that limits the standbycurrent used by the first inverter. Further, the second inverter iscoupled to an output of the first inverter. The input buffer converts afirst logic level of an input signal provided to the first inverter to asecond logic level at an output of the second inverter. The buffercircuit also includes a second current limiting circuit that is coupledbetween the first and second inverters to further limit the standbycurrent in the buffer circuit.

In one embodiment, the first current limiting circuit of the buffercircuit comprises a transistor that receives a feedback signal from theoutput of the second inverter to limit the standby current of the buffercircuit. In another embodiment, the first current limiting circuit ofthe buffer circuit comprises first and second transistors each having agate coupled to receive the output signal of the second inverter suchthat the first transistor limits the standby current of the firstinverter when the output of the buffer is a low logic level and thesecond transistor limits the standby current of the buffer when theoutput of the buffer is a high logic value.

In one embodiment, the second current limiting circuit of the buffercircuit comprises a current limiting transistor that is controlled by areference voltage. The current limiting transistor includes a drain thatis coupled to the output of the first inverter and the input of thesecond inverter in a standby current path for the buffer circuit. Thereference voltage is coupled to the source of the current limitingtransistor. The reference voltage thus establishes a low gate to sourcevoltage for the current limiting transistor so as to limit the standbycurrent of the buffer circuit. In another embodiment, the second currentlimiting circuit further comprises a voltage generator circuit thatgenerates a reference voltage that is approximately equal to a logiclevel of the input signal. In another embodiment, the second currentlimiting circuit further comprises a pass gate transistor that iscoupled between the current limiting transistor and the referencevoltage. The pass gate transistor passes the reference voltage to thesource of the current limiting transistor based on a feedback signalfrom the output of the second inverter. In another embodiment, thesecond current limiting circuit further comprises a circuit that delaysthe feedback signal when turning off the pass gate transistor to improvethe stability of the output of the buffer circuit.

In another embodiment, the second current limiting circuit comprisesfirst and second current limiting transistors. The first currentlimiting transistor is coupled to the output of the fist inverter tolimit the standby current when a low logic level is applied to the firstinverter. Further, the second current limiting transistor is coupled tothe output of the first inverter to limit the standby current when ahigh logic level is applied to the first inverter.

In another embodiment, the present invention provides a memory devicethat includes a plurality of buffer circuits. The memory device is usedin conjunction with an electronic system. The buffer circuits arecoupled to receive input signals from the electronic system. Each buffercircuit includes first and second inverters. The inverters are coupledtogether so as to convert a first logic level of the input signalprovided to the first inverter to a second logic level at an output ofthe second inverter. Each buffer circuit further includes a currentlimiting circuit that is coupled between the first and second invertersto limit the standby current in the buffer. The memory device furtherincludes: a control circuit that receives control signals from theelectronic system through the buffer circuits to control the operationof the memory device; an array of cells that store data for theelectronic system; and an addressing circuit that receives addresssignals from the electronic system that identify a cell in the array ofcells to be accessed by the electronic system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are block diagrams of illustrative embodiments of thepresent invention; and

FIGS. 2A and 2B are schematic diagrams of another embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific illustrative embodiments in which theinvention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that logical, mechanical and electrical changes may be madewithout departing from the spirit and scope of the present invention.The following detailed description is, therefore, not to be taken in alimiting sense.

FIG. 1A is a block diagram of an illustrative embodiment of the presentinvention. Buffer circuit 10 converts the logic level of an input signalwith a lower standby current than conventional buffer circuits. Buffercircuit 10 includes first and second inverters 12 and 14, respectively,and current limiting circuit 16. First inverter 12 receives an inputsignal, V_(in). An output of inverter 12 is coupled to current limitingcircuit 16. Current limiting circuit 16 is coupled to an input of secondinverter 14. The output of second inverter 14 is coupled to currentlimiter 18 of first inverter 12 through a feedback control input.

In operation, buffer 10 coverts the logic level of the V_(in) signal.For example, in one embodiment, buffer circuit 10 converts an inputsignal from a transistor-transistor logic (TTL) circuit to acorresponding signal used in devices using a complimentarymetal-oxide-semiconductor (CMOS) design.

To convert a low TTL logic level to a corresponding CMOS logic level,first inverter 12 receives the low TTL signal and produces an outputthat is approximately equal to the supply voltage. Current limiter 18 ofinverter 12 limits the standby current caused by the 0.7 to 0.8 voltageof V_(in) in first inverter 12. Second inverter 14 inverts the high CMOSlogic level from first inverter 12 to produce a low logic output that isapproximately at ground potential. Current limiter 18 uses this low CMOSlogic level to limit the current in first inverter 12. Current limitingcircuit 16 further limits the standby current of buffer circuit 10 byestablishing a low current on the order of 2 to 5 microamps throughconductive components of buffer circuit 10 at both high and low logiclevels.

FIG. 1B is a block diagram of a dynamic random access memory device 11that includes a number of buffer circuits 10′ for use with an electronicsystem 13. A number of buffer circuits 10′ may be constructed as shownand described -with respect to FIG. 1A so as to limit the standbycurrent used by memory device 11.

Electronic system 13 communicates with memory device 11 throughinput/output (I/O) lines, address lines and control lines. Electronicsystem 13 comprises, for example, a computer or other appropriate systemthat interfaces with a memory device. Buffer circuits 10′ are coupled tothe input/output lines, the address lines and the control lines asneeded to convert logic levels of signals. Memory device 11 comprises,for example, a dynamic random access memory. Memory device 11 furtherincludes control circuit 15, addressing circuit 9 and array of memorycells 17 that are constructed as known to a person of ordinary skill inthe art.

In operation, buffers 10′ convert logic levels of signals fromelectronic system 13 for use by memory device 11 with reduced standbycurrent. Electronic system 13 produces address, control and inputsignals using, for example, TTL logic levels. Buffers 10′ convert thelogic levels, for example, to CMOS levels for control circuit 15 andaddressing circuit 9 so as to store or retrieve data from array ofmemory cells 17.

FIG. 2A is a schematic diagram of another illustrative embodiment of thepresent invention. Buffer circuit 10 a includes first inverter 12 a,current limiting circuit 16 a, and second inverter 14 a.

First inverter 12 a includes p-channel transistor 20 and n-channeltransistor 22 coupled to form an inverter. An input voltage, V_(in), iscoupled to a gate of each transistor 20 and 22. A drain of transistor 20is coupled to a drain of transistor 22 to provide an output for inverter12 a. Further, inverter 12 a includes p-channel transistor 24 andn-channel transistor 26.

Transistors 24 and 26 limit the standby current in inverter 12 a.Transistors 24 and 26 each have a gate. The gates of transistors 24 and26 are coupled to receive the output of second inverter 14 a as afeedback control signal. A drain of transistor 26 is coupled to a sourceof transistor 22. A source of transistor 26 is coupled to ground.Similarly, a drain of transistor 24 is coupled to a source of transistor20 and a source of transistor 24 is coupled through a p-channel enabletransistor 28 to a voltage supply, V_(CC). Transistors 24 and 26 limitthe standby current by eliminating current paths in response to low andhigh logic levels for V_(in). For example, when V_(in) is a low logiclevel, second inverter 14 a outputs approximately zero volts whichprovides approximately zero volts from the gate to the source oftransistor 26. This eliminates transistor 26 as a standby current path.Similarly, when V_(in) is high, the output of second inverter 14 aprevents transistor 24 from conducting a significant amount of current.

Enable transistor 28 receives a signal, ENABLE1, that places buffercircuit 10 a in active-standby mode when ENABLE1 is a low logic level. Agate of enable transistor 28 is coupled to ENABLE1. A source of enabletransistor 28 is coupled to V_(CC) and a drain of enable transistor 28is coupled to the source of transistor 24. A second enable transistor 30includes a gate that is coupled to receive the ENABLE1 control signal.Further, a source of transistor 30 is coupled to ground and a drain oftransistor 30 is coupled to the output of inverter 12 at node A.

Current limiting circuit 16 a includes p-channel transistor 32 and 34that are placed in standby current paths for buffer circuit 10 a tolimit the standby current. The gates of transistors 32 and 34 arecoupled to receive the input signal, V_(in). The drains of transistors32 and 34 are coupled together at node A. Further, the drains oftransistors 32 and 34 are coupled to the drains of pass gate transistors36 and 38, respectively.

Pass gate transistors 36 and 38 are controlled by a logic circuit 40that has outputs coupled to the gates of transistors 36 and 38.Specifically, logic circuit 40 comprises NAND-gate 42 and first andsecond delay circuits 44 and 46. NAND-gate 42 receives an enable signal,ENABLE2, at a first input and a feedback control signal at a secondinput. As shown, the feedback control signal comprises the output fromsecond inverter 14 a. Other signals in buffer circuit 10 a can be usedin place of the output of second inverter 14 a with proper modificationof logic circuit 40. The output of NAND-gate 42 is coupled to delaycircuits 44 and 46. Delay circuit 44 provides a control signal to thegate of transistor 36 and delays only signals from NAND-gate 42 on apositive going signal. Finally, delay circuit 46 is coupled to the gateof transistor 38 and delays only signals from NAND-gate 42 on a negativegoing signal.

Current limiting circuit 16 a uses two reference voltages to control thestandby current of buffer circuit 10 a. The first reference voltage,REF1, is coupled to a source of pass gate transistor 38 through switch47. The second reference voltage, REF2, is coupled to a source of passgate transistor 36 through a switch 49. Voltages REF1 and REF2 areprovided by reference voltage generator 41. When switches 47 and 49couple voltages REF1 and REF2 to transistors 38 and 36, respectively,buffer circuit 10 a operates with low standby current. When switches 47and 49 couple voltages V_(CC) and ground to transistors 38 and 36,respectively, buffer circuit 10 a operates more conventionally, withhigher standby current.

Transistors 50 and 48 selectively apply the reference voltages REF1 andREF2 to transistors 38 and 36, respectively. The gates of transistors 48and 50 are controlled by a feedback signal from the output of secondinverter 14 a. Transistors 48 and 50 provide V_(CC) and ground prechargelevels for REF2 and REF1, respectively.

In operation, buffer circuit 10 a converts a signal from firstelectronic system 19 to a corresponding logic level for secondelectronic system 21. In the example described herein, first electronicsystem 19 is a TTL circuit and second electronic system 21 is a CMOSsystem.

When V_(in) goes from a low logic level to a high logic level, theinverter formed by transistors 20 and 22 begins to force the voltage atnode A to decrease toward ground. Once the voltage at node A decreasespast a threshold for inverter 14 a, the output of inverter 14 aincreases to produce a CMOS high logic output that is approximatelyequal to V_(CC). Thus, buffer circuit 10 a converts the TTL high voltageof 2.0 volts to a CMOS logic high that is approximately equal to thepower supply, e.g., 3.0 to 5.0 volts.

Buffer circuit 10 a converts the voltage level with reduced standbycurrent when compared to conventional buffers. The reduction in standbycurrent is significant when compared to conventional buffers. Transistor24 acts as a current limiter when V_(in) comprises a high logic level.The feedback signal from inverter 14 a provides a voltage to the gate oftransistor 24 that is approximately equal to the power supply voltage.Thus, transistor 24 is turned “off” eliminating a standby current path.For purposes of this specification, the term “off” means that thetransistor conducts an insignificant amount of current from drain tosource. Conversely, the term “on” refers to a transistor that conductsmore than an insignificant amount of current from drain to source. Thefeedback signal also turns on transistor 26 producing a current path forstandby current.

Transistor 32 controls the level of standby current that is conducted bytransistor 26 in response to a high logic input from first electronicsystem 19. When the output of inverter 14 a goes to a CMOS high logiclevel, approximately equal to the supply voltage, transistor 48 isturned off and transistor 50 is turned on. Transistor 48 thus couplesthe voltage REF2 to transistor 36 and transistor 50 couples transistor38 to ground. By coupling the source of transistor 38 to ground, currentlimiting circuit 16 a “precharges” node A to a low logic level. Logiccircuit 40 turns on pass gate transistor 36 and, after a delay, turnsoff transistor 38 to insure stability of buffer circuit 10 a. Pass gatetransistor 36 passes the REF2 voltage to the source of transistor 32.The value of the REF2 voltage is selected to set the current throughtransistor 32. For example, the REF2 voltage may exceed the high logicinput of V_(in) by approximately the threshold voltage of p-channeltransistor 36. Thus, the gate to source voltage of transistor 32 is lowand a low standby current is established in the current path includingtransistors 36, 32, 22, and 26.

In a similar manner, transistor 34 controls the level of standby currentthat is conducted by buffer circuit 10 a in response to a low logicinput from first electronic system 19. When the output of inverter 14 agoes to a CMOS low logic level approximately equal to ground potential,transistor 48 is turned on and transistor 50 is turned off. Transistor48 thus couples transistor 36 to V_(CC) and transistor 50 couplestransistor 38 to REF1. By coupling the source of transistor 36 toV_(CC), current limiting circuit 16 a “precharges” node A to a highlogic level. Logic circuit 40 turns on pass-gate transistor 38 and,after a delay, turns off pass-gate transistor 36. Pass gate transistor38 passes the REF1 voltage to the source of transistor 34. By selectingthe REF1 voltage, the current through transistor 34 is set For example,the REF1 voltage may be set to substantially equal to the low logicvalue of V_(in). Thus, the gate to source voltage of transistor 32 islow and a low standby current is established in the current pathincluding transistors 28, 24, 20, 34, and 38.

It is noted that in the description above, REF2 establishes a gate tosource voltage for transistor 32 when V_(in) is high that is differentfrom the gate to source voltage established by REF1 on transistor 34when V_(in) is low. In the exemplary embodiment, the REF2 voltage setsthe gate to source voltage of transistor 32 to a value of approximately0.7 to 0.8 volts-whereas the REF1 voltage sets the gate to sourcevoltage of transistor 34 to approximately zero volts. This difference ingate to source voltage does not unduly hinder the operation of buffercircuit 10 a. In the present embodiment, the difference in gate tosource voltages is selected so as to limit the swing in voltage requiredat the source of transistor 32 when V_(in) goes to a high TTL logicvalue. The TTL logic levels are not symmetric with respect to thecorresponding CMOS high and low logic values. By using the specifiedREF1 and REF2 voltages, buffer circuit 10 a maintains acceptable speedwhile also improving the standby current. In other embodiments, the gateto source voltages used for transistors 32 and 34 to limit the standbycurrent of the buffer circuit may both be set at other levels dependingon, for example, the nature of the input voltage, V_(in), the speedrequirements for the buffer circuit, and the acceptable level of standbycurrent.

Delay circuits 44 and 46 in logic circuit 40 improve the stability ofbuffer circuit 10 a by reducing oscillation in the voltage at node Aduring transitions in the voltage from system 19. For example, delaycircuit 46 delays turning off transistor 38 when the output of system 19goes from a low logic level to a high logic level. Thus, transistor 38provides a low voltage to the source of transistor 34 to keep transistor34 on and produce a low voltage at node A. If transistor 38 were turnedoff without a delay, transistor 32 could try to pull node A to a higherlogic level thus producing an oscillation in the output. Similarly,delay circuit 44 aids in preventing oscillation when the input fromsystem 19 goes from a low logic level to a high logic level.

FIG. 2B is a schematic diagram of a reference voltage generatorindicated generally at 41 a for use in buffer circuit 10 a of FIG. 2A.Generator 41 a includes n-channel transistor 52 with a gate that iscoupled to a control signal, LOWVCC*. The control signal LOWVCC*establishes one of two modes of operation for reference voltagegenerator 41 a.

Four diode coupled p-channel transistors 54 a through 54 d are coupledin series from a drain of transistor 52. Further, the substrate of eachtransistor 54 a through 54 d is coupled to the supply voltage. Fourn-channel diode coupled transistors 56 a through 56 d are coupled inseries between the source of transistor 52 and ground. The gates oftransistors 54 b through 54 d are coupled through switches 58 b through58 d, respectively, to the REF2 node. Similarly, the gates oftransistors 56 b through 56 d are coupled through switches 62 b through62 d to the REF1 node.

Transistors 60 and 66 establish the mode of operation for generator 41a. Transistor 60 has a drain that is coupled to the REF2 node. The gateof transistor 60 is coupled to LOWVCC* and the source of transistor 60is coupled to the supply voltage, V_(CC). Transistor 66 has a gate thatis coupled to receive the inverted LOWVCC* signal from inverter 64. Thesource of transistor 66 is coupled to ground and the drain of transistor66 is coupled to the REF1 node.

In operation, generator 41 a generates two reference voltages forcurrent limiting circuit 16 a of FIG. 2A. The LOWVCC* control signalplaces reference voltage generator 41 a in one of two modes ofoperation. First, when V_(CC) is above a threshold, LOWVCC* produces avoltage that turns on transistors 60 and 66. Thus, REF2 is approximatelyequal to V_(CC) and REF1 is approximately at ground potential. Further,when V_(CC) is above the threshold, LOWVCC* produces a voltage thatturns off both transistors 60 and 66 so that the voltage established bytransistors 54 a through 54 d produces the voltage for REF2 andtransistors 56 a through 56 d produce the voltage for REF1.

When V_(CC) is above the threshold, LOWVCC* establishes a currentthrough transistors 54 a through 54 d and transistors 56 a through 56 d.This establishes potential reference voltages for REF2 and REF1 at thegates of transistors 54 b through 54 d and transistors 56 b through 56d, respectively. To select the voltage at the gate of transistor 54 d,switch 58 d is closed and switches 58 b and 58 c are opened. Thus REF2is approximately one diode drop below the power supply. In otherembodiments, switches 58 b and 58 c can be used to establish differentvoltages for REF2. Similarly, switches 62 b through 62 d establish thevoltage for REF1 to be a selected number of diode drops above groundpotential.

Conclusion

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. This application isintended to cover any adaptations or variations of the presentinvention. For example, buffer circuit 10 can be modified to convertlogic levels other than from TTL to CMOS. Further, other values may beused for REF1 and REF2 to meet specific standby current levels. Further,REF1 and REF2 can be modified to meet changes in supply voltage, logiclevels and other parameters of the buffer circuit. Other referencevoltage generators may be used; even reference voltage generators thatdo not provide switchable voltage levels as shown in FIG. 2B.

The embodiments shown in FIGS. 2A and 2B include exemplary sizing forthe various transistors. Next to each transistor, the width to lengthfor the transistor in the exemplary embodiment. It is noted that thesize of these transistors is shown by way of example and not by way oflimitation. The sizes can be varied to meet requirements for a specificelectronic system.

What is claimed is:
 1. A buffer circuit, comprising: a first inverterwith a first current limiter that limits the standby current used by thefirst inverter; a second inverter coupled to an output of the firstinverter so as to convert a first logic level of an input signalprovided to the first inverter to a second logic level at an output ofthe second inverter; and a second current limiting circuit coupledbetween the first and second inverters to further limit the standbycurrent in the buffer.
 2. The buffer of claim 1, wherein the secondcurrent limiting circuit comprises first and second current limitingtransistors, the first current limiting transistor coupled to the outputof the first inverter to limit the standby current when a low logiclevel is applied to the first inverter and the second current limitingtransistor coupled to the output of the first inverter to limit thestandby current when a high logic level is applied to the firstinverter.
 3. A buffer circuit comprising: a first inverter with a firstcurrent limiter that limits the standby current used by the firstinverter; a second inverter coupled to an output of the first inverterso as to convert a first logic level of an input signal provided to thefirst inverter to a second logic level at an output of the secondinverter; a second current limiting circuit coupled between the firstand second inverters to further limit the standby current in the buffer;and wherein the first current limiter comprises a transistor thatreceives a feedback signal from the output of the second inverter tolimit the standby current of the buffer circuit.
 4. The buffer of claim3 wherein the first current limiting circuit comprises first and secondtransistors each having a gate coupled for receiving the output signalof the second inverter such that the first transistor limits the standbycurrent of the first inverter when the output of the buffer is a lowlogic level and the second transistor limits the standby current of thebuffer when the output of the buffer is a high logic value.
 5. A buffercircuit, comprising: a first inverter with a first current limiter thatlimits the standby current used by the first inverter; a second invertercoupled to an output of the first inverter so as to convert a firstlogic level of an input signal provided to the first inverter to asecond logic level at an output of the second inverter; and a secondcurrent limiting circuit coupled between the first and second invertersto further limit the standby current in the buffer, wherein the secondcurrent limiting circuit comprises a current limiting transistor with adrain coupled to the output of the first inverter and the input of thesecond inverter in a standby current path for the buffer circuit andwherein a reference voltage is coupled to the source of the currentlimiting transistor that establishes a low gate to source voltage forthe transistor so as to limit the standby current.
 6. The buffer ofclaim 5, wherein the second current limiting circuit further comprises avoltage generator circuit that generates a reference voltage that isapproximately equal to a logic level of the input signal.
 7. The bufferof claim 6, wherein the voltage generator circuit produces a scalablereference voltage with a number of diodes coupled to a source of atransistor that biases the diodes to establish a plurality of selectablereference voltages.
 8. The buffer of claim 5, wherein the second currentlimiting circuit further comprises a pass gate transistor coupledbetween the current limiting transistor and the reference voltage forpassing the reference voltage to the source of the current limitingtransistor based on a feedback signal from the output of the secondinverter.
 9. The buffer of claim 8, wherein the second current limitingcircuit further comprises a circuit that delays the feedback signal whenturning off the pass gate transistor to improve the stability of theoutput of the buffer circuit.
 10. A method for converting logic levels,comprising the steps of: applying an input signal to a first inverter ofa buffer circuit that inverts the input signal, the first inverterincluding a first current limiter that limits the standby current usedby the first inverter; applying the inverted input signal to a secondinverter of the buffer circuit that is coupled to an output of the firstinverter so as to convert a first logic level of the input signalprovided to the first inverter to a second logic level at an output ofthe second inverter; and limiting the standby current in the buffercircuit with a second current limiting circuit coupled between the firstand second inverters.
 11. The method of claim 10, wherein the step oflimiting the standby current comprises the step of limiting the standbycurrent with first and second transistors, the first transistor coupledfor limiting the standby current when the first input signal comprises afirst logic level, and the second transistor coupled for limiting thestandby current when the input signal comprises a second logic level.12. A buffer circuit, comprising: a first inverter; a second invertercoupled to an output of the first inverter so as to convert a firstlogic level of an input signal provided to the first inverter to asecond logic level at an output of the second inverter; a currentlimiting circuit coupled to the output of the first inverter and to theinput of the second inverter to limit the standby current in the buffer,and wherein the current limiting circuit is controlled by a feedbacksignal from the output of the second inverter.
 13. A buffer circuit,comprising: a first inverter with a first current limiter that limitsthe standby current used by the first inverter; a second invertercoupled to an output of the first inverter so as to convert a firstlogic level of an input signal provided to the first inverter to asecond logic level at an output of the second inverter; a second currentlimiting circuit coupled between the first and second inverters tofurther limit the standby current in the buffer and wherein the firstcurrent limiter receives a feedback signal from the output of the outputof the second inverter.
 14. The buffer of claim 13, wherein the secondcurrent limiting circuit comprises first and second current limitingtransistors, the first current limiting transistor coupled to the outputof the first inverter to limit the standby current when a low logiclevel is applied to the first inverter and the second current limitingtransistor coupled to the output of the first inverter to limit thestandby current when a high logic level is applied to the firstinverter.
 15. The buffer of claim 14 wherein the first current limitingcircuit comprises first and second transistors each having a gatecoupled for receiving the output signal of the second inverter such thatthe first transistor limits the standby current of the first inverterwhen the output of the buffer is a low logic level and the secondtransistor limits the standby current of the buffer when the output ofthe buffer is a high logic value.
 16. A buffer circuit, comprising: afirst inverter: a second inverter coupled to an output of the firstinverter so as to convert a first logic level of an input signalprovided to the first inverter to a second logic level at an output ofthe second inverter; a current limiting circuit coupled to the output ofthe of the first inverter and to the input of the second inverter tolimit the standby current in the buffer; and wherein the currentlimiting circuit is controlled by the feedback signal from the output ofthe second inverter.
 17. The buffer circuit of claim 16 wherein thecurrent limiting circuit further comprises a voltage generator circuitthat generates a reference voltage that is approximately equal to thelogic level of the input signal.
 18. The buffer circuit of claim 17wherein the voltage generator circuit produces a scalable referencevoltage with a number of diodes coupled to a source of a transistor thatbiases the diodes to establish a plurality of selectable referencevoltages.
 19. The buffer circuit of claim 16 wherein the currentlimiting circuit further comprises a pass gate transistor coupledbetween the current limiting transistor and the reference voltage forpassing the reference voltage to the source of the current limitingtransistor based upon a feedback signal from the output of the secondinverter.
 20. The buffer circuit of claim 16 wherein the currentlimiting circuit further comprises a circuit that delays the feedbacksignal when turning off the pass gate transistor to improve thestability of the output of the buffer circuit.
 21. The buffer of claim16 wherein the current limiting circuit comprises first and secondtransistors each having a gate coupled for receiving the output signalof the second inverter such that the first transistor limits the standbycurrent of the first inverter when the output of the buffer is a lowlogic level and the second transistor limits the standby current of thebuffer when the output of the buffer is a high logic value.
 22. Thebuffer of claim 21, wherein the current limiting circuit furthercomprises a voltage generator circuit that generates a reference voltagethat is approximately equal to a logic level of the input signal. 23.The buffer of claim 22, wherein the voltage generator circuit produces ascalable reference voltage with a number of diodes coupled to a sourceof a transistor that biases the diodes to establish a plurality ofselectable reference voltages.
 24. The buffer of claim 16, wherein thecurrent limiting circuit further comprises a pass gate transistorcoupled between the current limiting transistor and the referencevoltage for passing the reference voltage to the source of the currentlimiting transistor based on a feedback signal from the output of thesecond inverter.
 25. The buffer of claim 24, wherein the currentlimiting circuit further comprises a circuit that delays the feedbacksignal when turning off the pass gate transistor to improve thestability of the output of the buffer circuit.